Optical interferometry is frequently used to measure extremely small distances. Conventional single wavelength interferometric measurement does not extend well beyond about one optical wavelength. Absolute Distance Interferometry (ADI) is a technique for measuring distances greater than one wavelength. Two interferometers, each operating at two or more wavelengths, are used in ADI. ADI calibrates measurements in an interferometer of unknown length by comparison with a reference interferometer of known length. ADI determines distances from the ratio of the phase change induced across multiple frequencies, which is proportional to the ratio of the interferometer lengths.
The optical path difference of an interferometer of unknown length, D, is compared with the optical path difference of a reference interferometer of known length, L, by illuminating both interferometers with the same laser and tuning the laser over a frequency interval, Δf. The phase change, ΔΘ, induced for the interferometer of unknown length, D, is given as:
  ΔΘ  =                    2        ⁢        π            c        ⁢    D    ⁢                  ⁢    Δ    ⁢                  ⁢    f  where c is the speed of light. The phase change induced for the reference interferometer, ΔΦ, is given as:
  ΔΦ  =                    2        ⁢        π            c        ⁢    L    ⁢                  ⁢    Δ    ⁢                  ⁢    f  The measured phase changes for both interferometers can be used to calculate the length, D, of the interferometer of unknown length:
  D  =      L    ⁢          ΔΘ      ΔΦ      If the phase change exceeds 2π, fringe counting must be applied in order to properly determine the total phase shift.